CVE-2021-41819: CGI::Cookie.parse in Ruby through 2.6.8 mishandles security prefixes in cookie names

This week

HIGH 7.5REMOTEWATCHLISTDOWNGRADEUPGRADEMITIGATE

CVE-2021-41819 is a high-severity flaw (CVSS 7.5) affecting cgi, ruby, software collections. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

CGI::Cookie.parse in Ruby through 2.6.8 mishandles security prefixes in cookie names. This also affects the CGI gem through 0.3.0 for Ruby.

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	3.2% (top 12%)
CISA exploited list	No
Weakness	CWE-565
Published	2022-01-01
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · redhat · ruby-lang · suse

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2022-01-01 06:15 UTCPublished CVE record published
2026-06-17 04:08 UTCUpdated Record last modified

ruby-lang cgi DOWNGRADE

Affected: 0.1.0 · 0.2.0 · 0.3.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 0.1.0

1. Check installed version

apt-cache policy cgi || rpm -q cgi || cgi --version

2. No fixed release: DOWNGRADE to a release before 0.1.0 (Debian / Ubuntu)

apt-cache madison cgi sudo apt install cgi=<VERSION_BEFORE_0.1.0> sudo apt-mark hold cgi

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list cgi sudo dnf downgrade cgi-<VERSION_BEFORE_0.1.0> sudo dnf versionlock add cgi

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

ruby-lang ruby UPGRADE

Affected: <= 2.6.8 · >= 2.7.0 and < 2.7.5 · >= 3.0.0 and < 3.0.3

Fixed in: 2.7.5, 3.0.3

Safe to downgrade to: a release before 2.7.0

1. Check installed version

apt-cache policy ruby || rpm -q ruby || ruby --version

2. UPGRADE to 2.7.5 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade ruby

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade ruby

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.7.0 (Debian / Ubuntu)

apt-cache madison ruby sudo apt install ruby=<VERSION_BEFORE_2.7.0> sudo apt-mark hold ruby

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list ruby sudo dnf downgrade ruby-<VERSION_BEFORE_2.7.0> sudo dnf versionlock add ruby

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

redhat software collections MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy software-collections || rpm -q software-collections || software-collections --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

redhat enterprise linux MITIGATE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

debian debian linux MITIGATE

Affected: 9.0 · 10.0 · 11.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

suse linux enterprise MITIGATE

Affected: 11.0 sp1 · 12.0 · 15.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 11.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-41819?

cgi: downgrade or apply the listed mitigation; ruby: upgrade to 2.7.5, 3.0.3; software collections: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-41819 being exploited?

CVE-2021-41819 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-41819 affect?

CVE-2021-41819 affects cgi, ruby, software collections. Severity is high (CVSS 7.5). CGI::Cookie.parse in Ruby through 2.6.8 mishandles security prefixes in cookie names. This also affects the CGI gem through 0.3.0 for Ruby.

How urgent is CVE-2021-41819?

Priority 63/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities · Ruby vulnerabilities

Related CVEs

CVE-2025-27220 MEDIUM 4.0 UPGRADE In the CGI gem before 0.4.2 for Ruby, a Regular Expression Denial of Service (ReDoS) vulnerability exists in t
CVE-2025-27219 MEDIUM 5.8 UPGRADE In the CGI gem before 0.4.2 for Ruby, the CGI::Cookie.parse method in the CGI library contains a potential Den
CVE-2021-41816 CRITICAL 9.8 UPGRADE MITIGATE CGI.escape_html in Ruby before 2.7.5 and 3.x before 3.0.3 has an integer overflow and resultant buffer overflo
CVE-2021-33621 HIGH 8.8 UPGRADE MITIGATE The cgi gem before 0.1.0.2, 0.2.x before 0.2.2, and 0.3.x before 0.3.5 for Ruby allows HTTP response splitting

References

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